2009-06-14T20:16:00.000-07:00

Dichorionic-Diamniotic

can be

Fraternal or Identical

The twin peak sign indicates the presence of a dichorionic-diamniotictwin gestation. The twin peak represents the extension ofplacental villi into the potential space that is formed fromthe reflection of apposed amniotic and chorionic layers fromeach fetus. It forms where two separate placentas growcontiguously and appear fused. The twin peak can be of variablesize, and only its presence is required to suggest that thepregnancy is dichorionic-diamniotic. This sign is most usefulin assessing the chorionicity of pregnancies after 10 weeks. Prior to this time, gestational sacs are readily recognizableand allow a rapid and accurate determination of chorionicity.

The above drawing illustrates how the twin peak sign is reliable evidence of dichorionicity. In a dichorionic pregnancy, both the amnions and the chorions reflect away from the placental surface, creating a potential space into which villi can grow. Large arrow shows the resulting twin peak sign. Monochorionic diamniotic pregnancies have a single layer of continuous chorion (large arrow) limiting villous growth; the apposed amnions form a thin membrane separating the two amniotic cavities. A = amnion, C = chorion.

Monochorionic/Diamniotic

are

Identical

Courtesy of The Department of Radiology, Beth Israel Deaconess Medical Center, Boston, MA.

Monochorionic/Diamniotic Twins (mono/di)

Identical twin pregnancy ultrasound Ultrasound of another pregnancy at same gestational age as above, but with identical (monozygotic) twins one gestational sac is seen with 2 yolk sacs visible inside it.
This results from an early embryo dividing into 2 genetically identical embryos.

5 Weeks Gestation

Monochorionic/Diamniotic Twins (mono/di)
The intertwin membrane is barely visible, look closely and you will see it just below the baby on the left.

10 Weeks Gestation

Monochorionic/Diamniotic Twins (mono/di)
Notice the membrane, it is only partially visible in this mono/di twin sonogram.

12 Weeks Gestation

Dichorionic/Diamniotic Twins

These ultrasound images demonstrate twin pregnancy at 13 weeks gestation. The intertwin membrane reveals that this is a diamniotic twin pregnancy. Also note the presence of the triangular wedge of placental tissue projecting into the intertwin membrane. This is called the Twin peak sign or the lambda sign which clearly suggests a Dichorionic twin pregnancy. Ultrasound images of Dichorionic gestation courtesy of Dr. Mohammed Sheriff, Tanzania.

13 Weeks Gestation

Dichorionic/Diamniotic Twins
Transverse image of the placenta of a twin pregnancy at 17 weeks gestational age reveals a triangular peak of villi (short arrow) extending from the placenta into the intertwin membrane (long arrow): the twin peak sign. Amniocentesis confirmed the pregnancy to be dizygotic, with male and female fetuses.

17 Weeks Gestation

Additional Information

Source Link

Journal of Ultrasound in Medicine, Vol 14, Issue 6 415-419, Copyright © 1995 by American Institute of Ultrasound in Medicine
JOURNAL ARTICLE
Using the number of yolk sacs to determine amnionicity in early first trimester monochorionic twins
B. Bromley and B. Benacerraf
Department of Obstetrics and Gynecology, Massachusetts General Hospital, Harvard Medical School, Boston, USA.
The purpose of this study was to evaluate the relationship between thenumber of yolk sacs and amnionicity in monochorionic twin pregnanciesscanned early in the first trimester. We retrospectively reviewed images ofall monochorionic twins scanned between 6 and 9.5 weeks' gestation and withpathologic or sonographic confirmation of chorionicity-amnionicity. Eachfilm was reviewed for the number of yolk sacs present, as well as for thegestational age at which the amniotic membrane was first visualized. Twentymonochorionic-diamniotic pregnancies and two monochorionic-monoamnioticpregnancies met the criteria for inclusion in the study. In diamnioticpregnancies scanned at less than 8 weeks' gestation, only the yolk sacswere identified; none of the dividing amniotic membranes were detected. Twoyolk sacs were identified in all but one case. In this case, although oneyolk sac was seen at 6 weeks, follow-up scanning at 8 weeks revealed twoyolk sacs. In each of the monochorionic-monoamniotic twin pregnancies, oneyolk sac was seen at 9 weeks and a single amnion encircled both embryos. Weconclude that the sonographic identification of two yolk scas inmonochorionic twins enables us to make the diagnosis of diamniotic twinsearly in the first trimester, before the amniotic membrane can be imaged.The presence of one yolk sac should prompt a follow-up ultrasonogram toassign amnionicity definitively.

So You're Pregnant With Twins?

JOURNAL ARTICLE

Using the number of yolk sacs to determine amnionicity in early first trimester monochorionic twins